There are times when I have to think hard about working with some clients. They maybe provide services or products that I am not sure about or work in ways that I disagree with. I have limits and do say no sometimes. But for the others I justify my involvement in the following way.
If you have a big enough, mature enough, and lively enough internal network maybe when someone suggests sub prime mortgages and enough of you go "Really?" they think again. Maybe if someone proposes putting yet more sugar in your prepared food product and enough of you go "Really?" they choose a healthier option. Maybe if your organisation screws up, tries to spin the situation, and enough of you go "Really?" they do the right thing and take it on the chin.
In my book I suggested that if our corporate behemoths are becoming harder to constrain from the outside maybe we could all play our parts in doing so from the inside.